2. Description of Prior Art
High voltage power systems of the future will have distributive generation systems of many different types that are more sensitive to overloads and surges. Use of better conducting wire will lessen system resistance and provide more current during disturbances, etc. When faults occur, we will get more damage at the fault and possibly more damage to sensitive generation and delivery equipment. Customer's loads have equipment that is more sensitive to surges and sympathetic voltage dips, so the duration of any disturbance will be critical. Loads will continue to increase and economics may require systems to operate at optimal limits, where loss of a piece of equipment or any kind of extended outage of any equipment or line may not be acceptable. Also, if we reduce the damage during a fault or overload, at the fault or to equipment, then that section may be restored sooner.
In existing systems of today we have intelligent relays and equipment that can sense overloads and surges very quickly and make decisions very quickly. However, their outputs usually operate a very big and slow circuit breaker which can only perform one function regardless of the disturbance. Circuit breaker operation is usually delayed to ensure we don't have a false alarm or to coordinate with other devices in the circuit, again because the breaker has such an abrupt action of interrupting everything. Further, if one of these circuit breakers fail or cannot be closed back in we may suffer an extended outage or load flow problem.
Some improvements would be gained by using a current limiter. This device limits the amount of current flowing through it for a short time until interrupters are opened or some other decision is made to by-pass the disturbance.
Prior art describe variable inductor type of current limiters, i.e. U.S. Pat. No. 5,930,095. On these the inductance can be varied by magnetizing the core through another source and changing its saturation level. The values of inductance in such a device can be very limited. It can be considered slow to react from one state to another and requires a continuous source of bias current to saturate the core under normal conditions.
Some newer technology patents such as U.S. Pat. Nos. 4,490,769, 6,337,785, 6,433,660, and 6,437,960 describe current limiters, employing a combination of semiconductor devices, PTC's, superconductor devices, and/or inductor coils. These types of limiters usually rely on the semiconductor, PTC, or superconductor to carry the full amount of the normal load current and when called upon to act under a high current fault condition for these devices to shunt away the current into an inductive coil or to become more resistive. Therefore, the devices have to be larger and slower. If one fails or does not return back to normal after operation, it does not allow normal current flow. Also, they can become excessively hot and this can affect their characteristics over time.
These “current limiter” types of current controllers that are described above are normally used to limit current in case of faults. They usually do it by switching from normal to one value of fixed impedance and can only withstand the fault current for a short period of time. Any controllers that can control or limit load flow usually suffer from similar deficiencies as the current limiters. For instance, controllers described in U.S. Pat. Nos. 5,530,613 and 5,723,915 have to have larger and good heat dissipating components. Also, if one of these components fail there could be current flow problems or the controller may not work.
In conclusion, the current controllers described above lack several things that could be very advantageous. Things such as: the ability to vary the impedance to different amounts depending on the disturbance, the ability to carry the overload for a longer period of time especially for load flow control, the ability to allow maintenance or repair/replacement of more sensitive components without having to take the whole device out of service, and, again, the ability to operate very fat and to continue functioning if one of these sensitive components fails.